Nova remnant GK Per

About this image
The first image is a four minute exposure taken
on the night of September 26th 1994 (UT of observation 27/09/94:07:21)
using a red-sensitive (R-band) filter. The brightness of the image has
been converted to color (a technique called pseudo-color) using
a simple orange/red-shading-to-white
transformation, purely to make a more attractive picture.
The second image is a combination of several exposures taken
on the night of October 11th 1994 (UT of observation 12/10/94:10:04 to
10:34) with the 1k detector.
Images were taken through three different filters approximating red (three
exposures for a total of four-and-a-half minutes), blue (three, total six
minutes) and green (three, total six minutes).
The individual colors were aligned and combined in the computer
to create this (approximately) true color picture.
Each photograph shows a region 120 arc seconds square.
The approximate "seeing" is 1.2 arc seconds for the first image, and
1.0 arc seconds for the color image.

About this object
The unusual nova shell GK Per is the result of Nova Persei 1901, a nova which
exploded in 1901 about 1500 light years away in the direction of the
constellation Perseus. Classical novae, nowadays categorized as cataclysmic
variables, normally comprise a hot white dwarf with accretion disk, and
a cool mass-transferring companion. At some point, hydrogen burning triggers
thermonuclear runaway in the accreted matter, leading to an explosive shock
wave and an expanding shell.

The first bright nova of the 20th century and the first to be subjected
to detailed spectroscopy and photometry, Nova Persei 1901 reached a maximum
brightness of 0.2m and declined very rapidly thereafter to its current minimum
of 13.1. GK Per is unique among classical novae, having the longest known
period (almost two days) and showing dwarf nova-like outbursts of about 3 mag.
It also contains an evolved secondary (type K2IV), while all others have
main sequence companions.

Expanding nebulosity was detected as early as 1902, and apparent superlight
speeds were explained as reflection from dust grains, which has only been
seen in one other nova. The ejecta show an asymmetry unique among novae,
contain as much as one ten-thousandth of a solar mass, and have speeds
reaching 1200 km/s. The unusual features of this `firework burst' nebula
are best explained by expansion into an interstellar medium that is
considerably denser than
the average, although it is not yet clear why this should be so.